In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities, there has been and continues to be efforts toward scaling down device dimensions to submicron levels (e.g., below 0.35 microns) on semiconductor substrates. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of metal interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features. Conventionally, analog precision and mixed signal devices have not been fabricated employing these submicron densities. This is because the precision of the analog devices and the selection of available materials for precision analog devices have been overriding factors over device density and device speed. However, with the increased importance of reduced size and increased speeds in analog applications, and the increased integration of digital and analog devices in substrates, there is an increased desired to employ analog devices in submicron processes.
Conventionally, doped polysilicon is employed as a material of a resistor in a semiconductor fabrication. However, the resistance of a doped polysilicon resistor is controlled by the size of the predetermined length and area of the doped polysilicon layer. Therefore, to increase the resistance per unit of a resistor, thin film resistor materials are employed such as silicon chromium (SiCr) alloy, nickel chromium (NiCr) alloy, tantalum nitride, titanium nitride, and tungsten. Thin film resistors are very attractive components for high precision analog and mixed signal applications. In addition to a low thermal coefficient of resistance and low voltage coefficient of resistance, thin film resistors provide good resistor matching and good stability under stress.
High frequency mixed signal applications require the use of metal interconnects. Integrated circuit metal interconnects are formed using damascene processes. In a damascene process a trench is first formed in a dielectric layer. The trench is then filled with metal and the excess metal is removed by a number of different techniques, including chemical mechanical polishing.
The formation of thin film resistors in an integrated circuit containing metal interconnects presents many challenges. The thin film resistor is not formed using metal interconnect material and, therefore, is incompatible with existing damascene processes. The incompatibility is exacerbated by the requirement that the thin film resistors be formed in the same levels as the metal interconnects.